Ice-repellent coatings can have significant impact on improving safety in many infrastructure, transportation, and cooling systems. Among numerous problems caused by icing, many are due to striking of supercooled water droplets onto a solid surface. Such icing caused by supercooled water, also known as freezing rain, atmospheric icing, or impact ice, is notorious for glazing roadways, breaking tree limbs and power lines, and stalling airfoil of aircrafts.
When supercooled water impacts surfaces, icing may occur through a heterogeneous nucleation process at the contact between water and the particles exposed on the surfaces. Icing of supercooled water on surfaces is a complex phenomenon, and it may also depend on ice adhesion, hydrodynamic conditions, the structure of the water film on the surface, and the surface energy of the surface (how well the water wets it). The mechanism of heterogeneous ice nucleation on inorganic substrates is not well understood.
Melting-point-depression fluids are well-known as a single-use approach that must be applied either just before or after icing occurs. These fluids (e.g., ethylene or propylene glycol) naturally dissipate under typical conditions of intended use (e.g. aircraft wings, roads, and windshields). These fluids do not provide extended (e.g., longer than about one hour) deicing or anti-icing. Similarly, sprayed Teflon® or fluorocarbon particles affect wetting but are removed by wiping the surface. These materials are not durable.
Chemical character of a surface is one determining factor in the hydrophobicity or contact angle that the surfaces demonstrate when exposed to water. For a smooth untextured surface, the maximum theoretical contact angle or degree of hydrophobicity possible is about 120°. Surfaces such a polytetrafluoroethylene or polydimethylsiloxane are examples of common materials that approach such contact angles.
Recent efforts for developing anti-icing or ice-phobic surfaces have been mostly devoted to utilize lotus leaf-inspired superhydrophobic surfaces. These surfaces fail in high humidity conditions, however, due to water condensation and frost formation, and even lead to increased ice adhesion due to a large surface area.
Many investigators have produced high-contact-angle surfaces through combinations of hydrophobic surface features combined with roughness or surface texture. One common method is to apply lithographic techniques to form regular features on a surface. This typically involves the creation of a series of pillars or posts that force the droplet to interact with a large area fraction of air-water interface. However, surface features such as these are not easily scalable due to the lithographic techniques used to fabricate them. Also, such surface features are susceptible to impact or abrasion during normal use. They are single layers, which contributes to the susceptibility to abrasion.
Other investigators have produced coatings capable of freezing-point depression of water. This typically involves the use of small particles which are known to reduce freezing point. Single-layer nanoparticle coatings have been employed, but the coatings are not abrasion-resistant. Many of these coatings can actually be removed by simply wiping the surface, or through other impacts. Others have introduced melting depressants (salts or glycols) that leech out of surfaces. Once the leeching is done, the coatings do not work as anti-icing surfaces.
Nanoparticle-polymer composite coatings can provide melting-point depression and enable anti-icing, but they do not generally resist wetting of water on the surface. When water is not repelled from the surface, ice layers can still form that are difficult to remove. Even when there is some surface roughness initially, following abrasion the nanoparticles will no longer be present and the coatings will not function effectively as anti-icing surfaces.
In some applications, transparent coatings are very important. For example, transparency in functional coatings is desirable for residential and vehicle windows, optical lenses, filters, instruments, sensors, eyeglasses, cameras, satellites, weapon systems, and photovoltaic glass.
Yet, there are fundamental limitations prohibiting visual transparency. Integrating hydrophobicity and transparency within the same surface presents significant challenges. Hydrophobicity typically competes with transparency because the surface features (i.e., surface roughness) associated with hydrophobicity typically scatter light, making surfaces appear opaque or translucent. Additionally, surfaces with large roughness usually exhibit weak mechanical stability.
For example, polymer-based films are typically not bonded to the substrate well enough to be sufficiently durable for most application requirements. Powder-based coatings also exhibit weak durability. Sol-gel based coatings can offer better bonding; however, they generally exhibit poor hydrophobic qualities. Coatings based on nanoarrays have similar problems to polymer or sol-gel based films. Furthermore, fabrication of these nanostructure assemblies involves elaborate processing schemes that render them unsuitable for large-scale development and production.
There is a need in the art for scalable, impact-resistant, transparent coatings that have both dewetting and anti-icing properties. Such coatings preferably utilize low-cost, lightweight, and environmentally benign materials that can be rapidly (minutes or hours, not days) sprayed or cast in thin layers over large areas using convenient coating processes. These coatings should be able to survive environments during use over extended periods.